9. Not staying in touch with friends

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Outside of school and work, you're unlikely to wind up in situations where meeting people is purely incidental, where everyone is more or less forced to get to know one another and form friendships. Of course, you're not guaranteed to make friends in either situation, which is why it's so important to retain the ones you have. Neglecting friendships is easy — everyone's busy and everyone loses touch from time to time — but trying to revisit friendships after years have passed rarely works out.